It seems to me that one good reason to do so is that for all the ways that these works have been analyzed and surpassed in the intervening years, the reader can be sure that what is written there is not the product of manipulation by the forces that are at work in the reader's own time and place. So it represents another way to gain valuable freedom and distance.

Outside of learning about the context/history of some field of thought, I think that's the general reason people give for recommending "classic" works of nonfiction.

Older works can also differ in their presentation, which can make them more interesting. You bring up Euclid, so I feel I'm free to mention I really, really wish I'd learnt Calculus from a high-level text like Courant, instead of Stewart and Sallas+Hille+Etgen. I would've have become more enthusiastic about math much earlier. (Maybe. Ah, counterfactuals.)